10.15.4 - Target display mode no longer working (16' MBP with iMac'27 late 2009)

[htomi]

All,

I've spent the past 3 days trying to enable Target Display Mode on my iMac '27 late 2009.
After pressing the CMD +F2 on the iMac keyboard, the iMac screen goes black for few seconds, then it reverts back to the login/desktop screen.

I believe the issue is software related, however I cannot find the solution on my own.

Facts:
1. When I got my personal 16' MBP back in March'20, the target display mode was working under 10.15.1 and 10.15.2
2. TDM was working without issues with my previous company MBP (15' 2017 model) with 10.15 and 10.15.1
3. I've tried SMC, PRAM resets on both machines - no joy
4. As a test I've managed to install an 10.15.3 beta onto the 16' MBP and I was able to enable the TDM few times.
This also confirms that the adapter and DisplayPort Cable are both fine.
5. 10.15.3 beta behaves differently in case of unsuccessful connections, as the iMac screen goes black and stays black till the next CMD+F2 keypress, eg it does not revert to the login/desktop screen automatically compared to 10.15.4

I've tried to downgrade to 10.15.1 or 10.15.2 however the latest available OS from the AppStore is 10.15.4.

Machines:
16' MBP - OSX 10.15.4
27' iMac (late 2009) - OSX 10.13.6 (all available updates installed)

Thanks in advance!

 

[htomi]

I've tried to downgrade to 10.15.1 or 10.15.2 however the latest available OS from the AppStore is 10.15.4.= cannot downgrade to previous versions.

 

[teeck2000]

Hey just thought I chime in cause I've had a huge mess of display issues on my MBP 16" after upgrading(downgrade honestly) to 10.15.4 and been so frustrated I don't wont others going through it. I don't have good news though... nothing has fixed my external display working correctly and like you I couldn't downgrade cause they don't offer it but I had the 10.15.0 installer on my old 15" macbook pro and just made a new partition and installed 10.15.0 and updated to 10.15.2 and made a clean timemachine backup and used that to restore the MBP 16", cause booting from external to install no matter what I tried the apple server would not let me install from the external USB. BUT it didn't work at all, cause the firmware after getting updated on 10.15.4 breaks displays so no current workaround, cause you cant downgrade.

 

[htomi]

Thanks! I’ll wait for others to chime in. I’m suspecting the display driver as with the the 10.15.3 Beta I was able enabling the TDM few times.

 

[Julian Milo A.]

I am having the exact same issue. :'( anyone help?!
iMac 10,1 (27 inch late 2009 8 gb ram, installed on a ~500GB SSD)
It worked fine when my iMac was running high sierra, just had to plug it in and it would work. Now even when i enter cmd F2, the screen goes dark for a second and then the iMac desktop shows.
Thanks!

 

[htomi]

It seems, the firmware included 10.15.4 update is no longer compatible with many 3rd party adapters, hubs, docks and as a result, external displays or even the TDM with older iMacs aren’t recognized anymore. I’ve tested my alogic hub with a Lenovo machine- Win 10 and it works perfectly, however no display signal are detected when using it with the MBP. I ended up using the apple usb-c digital av adapter with hdmi cable to connect to my 27’ ViewSonic screen. The apple adapter is working fine. Some people reported positive results by adding an usb-c male to female cable between the Mac and 3rd party adapters.

 

[htomi]

No changes post the 10.15.5 update (ffs apple) (used the combo updater)
No display signal is detected when using the the alogic dock (ULDPLS-SGR - https://www.alogic.co/ir/catalog/pr...elivery-ultra-series-space-grey/category/262/).
The dock has 2x 3.1 USB ports, MicroSD, SD Card Reader, and Ethernet ports, these are still working as designed. (at least the expensive dock is not a paperweight)

Apple's own usb-c digital av adapter with hdmi cable is still working. (what a surprise...)

I'd love to use the old iMac as screen, but lost all interest (and not in a mood spending more $$) to experiment with any 3rd party docks, hubs etc.

 

[htomi]

No changes post the 10.15.5 supplemental update either.

 

[htomi]

No changes post the 10.15.6 update. The Mini Display Port & the HDMI port are both ‘dead’ on the hub when its connected to the 16’ MacBookPro. Both ports are working fine with a PC.